Allied Telesis AT-S97 - Manual (Page 74)

Chapter 5: Port and Module Commands
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This operating mode is useful when the network devices connected to the
ports of a channel can react to a loss of a link on a port, such as managed
Fast Ethernet switches running SNMP or a spanning tree protocol.
Conversely, the MissingLink mode will be of little value if the network
devices of a channel cannot react to a lost link. In the latter scenario, the
Link Test mode would probably be a better operating mode for a channel
during normal network operations.
Furthermore, Allied Telesis does not recommend using the MissingLink
mode when troubleshooting a network problem that may have its roots
with a link problem. The MissingLink mode will not allow you to use the
port’s Link LEDs or the SYSTEM SHOW INTERFACE command to
diagnose the problem, since neither port will show a link. Rather, the Link
Test and the Smart MissingLink modes are more useful when
troubleshooting a link problem.
Description of the Smart MissingLink Mode
The Smart MissingLink mode, the third operating mode of the media
converter channels, is nearly identical in terms of functionality to the
MissingLink mode. It, too, enables the two ports of a channel to share the
“Link” status of their connections.
The difference is rather than completely shutting off the transmitter of a
port when its companion port in a channel loses its link, this operating
mode pulses the port’s transmitter once a second. This signals the port’s
ability to still establish a link to its network device and that the loss of the
link originated on the companion port in the channel.
The advantage of this operating mode over the MissingLink mode is that
you can use the Link LEDs and the SYSTEM SHOW INTERFACE
command to troubleshoot a link failure with the ports of a channel. A port’s
Link LED starts to flash when its companion port cannot establish a link
with its network device and the port’s status changes to TX SML in the
SYSTEM SHOW INTERFACE command.
Here is an example of how the Smart MissingLink mode works. Assume
that the fiber optic port in a media converter channel lost its link to its
network device while the channel was in the Smart MissingLink operating
mode. The mode would respond by pulsing the transmitter on the twisted
pair port of the channel about once a second. The port’s Link LED would
flash and its status would change to TX SML in the SYSTEM SHOW
INTERFACE command as a signal that the failure originated on the fiber
optic port of the channel. When the connection is reestablished on the
fiber optic port, the twisted pair port resumes normal operations so that the
two ports can again forward traffic to each other.
The operating mode functions the same if the failure starts on the twisted
pair port. Here, the mode pulses the transmitter on the fiber optic port.